Push-pull modulator with adjustable control means



June 15, 1965 E. GQMILLIS ETAL PUSH-PULL MODULATOR WITH ADJUSTABLE CONTROL MEANS Original Filed July 3, 1958 3 Sheets-Sheet 1 R o i M m m H M m. i v G M m A G t .m u w b d o E R m N 5 0+ @3520: zocszfizl 5% l 212. L L N 5 5158 June 15, 1965 E. G- MILLIS ETAL 3,189,846

PUSH-PULL MODULATOR WITH ADJUSTABLE CONTROL MEANS ori inal Filed July :5; 1958 s Sheets-heet 2- A.C. OUTPUT Fig. 6

0.0. INPUT M). OUTPUT O 0.0. INPUT A.C. OUTPUT O Edwin G.M i|lis Robert A. Shearer INVENTOR.

Fig. 8

BY o.c. INPUT w June 15, 1965 E. e. MILLIS ETAL 3,189,846

PUSH-PULL MODULATOR WITH ADJUSTABLE CONTROL MEANS Original Filed July 3, 1958 3 Sheets-Sheet 3 Fig. I0 Edwin G. Millis Robert A.Shearer INVENTOR.

United States Patent 3,189,846 PUSH-PULL MODULATUR' WITH ADJUSTABLE CONTRGL MEANS Edwin G. Millis and Robert A.'Shea rer, Houston, Tern,

assignors to Texas Instruments Incorporated, Dallas Tern, a corporation of Delaware 2 Original application July 3, i958, Ser. No. 746,464, now Patent waaposnao, dated Nov, 7, 1961. Divided and this application Jan. '13, 1961, Ser. No. 88,592

' '2 Claims. (Cl. 332-37) The invention relates to direct-current amplifiers and more particularlyto a D.C.-to-A.C.-to-D.C. converter utilizing a modulator circuit.

This application is a division of patent application Serial No. 746,464, by the inventors hereof, filed July 3, 1958, now US. Patent No. 3,008,090, and assigned to the same assignee.

In accordance With the present invention direct current signals are applied to a' modulator having a carrier input derived from a temperature-stabilized oscillator. The output signals from the modulator are A.C. carrier signals, amplitude modulated in accordance with the amplitude of the DC. input signals. The alternating signals thus developed are amplified in an A.C. amplifier and demodulated by a demodulator, which develops a direct current signal which is an amplified version of the original direct current signal input. The demodulator circuit may, for eXample,-feed a galvanometer type recorder directly, and in accordance with one aspect of the present invention, the output impedance of the demodulator is relatively high, so as to provide for proper damping o th sa vatwm rme baa sm f he wa ds? The specific modulator employed has an A.C. output versus D.C. input voltage characteristic such that an A.C. output signal is produced even though no DC. input signal is applied thereto. In order to compensate for this modulator characteristic, the apparatus is provided with a bucking current generator which is adapted to produce a direct current in opposition to the direct current developed by the demodulator to eliminate deflections of the galvanorneter when the input signal attains zero value. The bucking current generator has high output impedance in order to provide appropriate damping for the galvanometer mechanism.

The modulator is provided with a circuit for shifting the operating range of the instrument, so that opera tion may be achieved over the linear portions of the operating range of the modulator, and further for modifying the operating range so that the modulator 'may respond to signals of one polarity only, or to signals of either polarity.

The specific modulator utilized with the apparatus of the invention employs transistors as switching devices, but since true open circuit operation of a transistor is difiicult of achievement, a bucking circuit is employed to eliminate the ellects of residual conduction through the transistors, Whennominally biased to cut-oil.

The oscillator employed to provide 'acarrier or A.C. input signalto the demodulator comprises a conventional phase-shift oscillator having a very simple, but highly erresuve temperature stabilization system. In accordance with this aspect of the invention, a transistor is employedas the active element of the oscillator and a primary winding of an output transformer is employed as the transistor load. A temperature sensitive'diode is placed in shunt with the primary winding of the transformer and provides a slight clipping action on one phase only of the output voltage. The diode is temperature sensitive and eile'ctivel'y increases its conduction with temperature as does the transistor, whereby the clipping effectbecomes'more pronounced as the temperature rises to compensate for increased transistor conductivity as temperature rises. In consequence, the

oscillator possesses negligible amplitude variation with temperature over the range 30 to F.

It is accordingly, an object of the invention to provide a modulator for an amplifier capable 'of drivinga galvanometer-type recorder havinglinearity of better'than 1% full scale. 1

It is'another object of the present invention to provide an amplifier for galvanometer-type recorders having a transistorized modulator including a circuit-for'com modulator and having, in addition, a'circuit-for com-' pensating for residual current how in transistor when in non-conductive condition.

' Other objects will become apparent from the following detailed description of a preferred'embodiment when taken with the drawings in which: I

FIGURE 1 is a schematic block diagram of a preferred embodiment of the amplifier of "the present invention; t I

FIGURE 2 is a schematic circuit diagram of a modulator circuit employed in the apparatus of the present invention; i

FIGURES 3 and 4 are equivalent circuit diagrams which are employed to explain the operation of the circuit of FIGURE'Z, i

FIGURE 5 is a schematic circuit diagram of a complete 'rnodulator circuit according to the presentinvention; I

FIGURES 6-8, inclusive, are plots of A.C. output voltage versus D.C. input voltage, employed in explaining the operation of the circuit of FIGURES;

FIGURE 9 is a schematic circuit diagram of the demodulator and bucking current generator of the' system of the present invention;and" p 4 FIGURE 10 is aschematic circuit diagram of an oscil lator employed in the system of the invention; 7

Referring specifically to FIGURE 1 of the accompanying drawings, which is a schematic block diagram of the amplifier system of the present invention, input'signals are applied via an input attenuator 1' to a modulator 2, having A.C.carrier signals-applied thereto from an oscillator 3. The A.C. signals developed in the modulator Z are amplified by an A.C. amplifier 4 and applied through a demodulator 5 to a galvanometer type recorder 6, representing 'an exemplary loadfor this system.

As will become apparent as the description of the various specific circuits of the system proceeds, the modulator 2 is of a type which produces an A.C. output signal Whether or not an input signal is applied thereto. In consequence, means are provided fOr'offsetting this undesirable efiect in the form of a bucking'currcnt generator 7, which provides a variable current in 0pposition to the current supplied by the demodulator'fi. The current supplied by 'the' generator '7"may be" varied at will, in order to reduce the input to the recorder to zero value when the DC. inputto meanest-nest is at zero potential. r

greases The apparatus illustrated in FIGURE 1, in order to be employed in-a high-quality instrument, must have a linear input versus output characteristic over substantially its entire operating range and must be insensitive to temperature and line voltage disturbances. The modulator of the present invention is particularly responsible for the linearity of the system, whereas the oscillator 3 is the element which primarily imparts required temperature stability to the system.

Reference is now made to FIGURE 2 of the accompanying drawings, by which means the operation of the modulator circuit 2 is explained more fully. The niodula tor circuit illustrated in FIGURE 2 comprises a pair of D.C. input terminals 8 and 9, with the terminal 8 connected via a lead 11 to an emitter electrode 12 of a first PNP-type transistor 13. The terminal 9 is connected via a lead 14- to an emitter electrode 16 of second PNP- type transistor 17. The transistor 13 is provided with a base electrode 18 and a collector electrode 19, which is connected to a collector electrode 21 of the transistor 17., The transistor 17 is further provided with a have electrode 22. The secondary Winding 23 of an input transformer 24 is connected between the base electrodes 18 and22 of the transistors 13 and 17, respectively. Thesecondary winding 23 of the transformer 24 is provided with a center tap 26, connected via a lead 27 to the collector electrodes 19 and 21 of the transistors 13 and 17, other end of the load resistor 28, as viewed in FIGURE respectively, and to one end of a load resistor 28 The other end of the lead resistor 28, as viewed in FIGURE 2, is connected to the lead 11 and, therefore, to the emitter electrode 12 of the transistor 13.

' The operation of the basic modulator circuit maybe explained by reference to FIGURES 3 and 4, which are equivalent circuit diagrams of the modulator in various conditions of its operation. Upon a positive excursion of the voltage applied to the base electrode 18 of the transistor 13' and a negative excursion of the voltage applied to thebase electrode 22 of the transistor 17, the transistor 17 becomes conductive and the transistor 13 becomes essentially non-conductive. During the period of conduction of the transistor 17, reference being now made specifically to FIGURE 3, it appears as a constant volt-j age source 29 while the transistor 13, being substantially cut-otf, appears as a constant. current: generator 31 of small output. The load impedance of the D.C. signal source applied across terminals 8 and 9 in FIGURE 2 is represented by the resistor 32, while the load resistor I bears the numeral 28, as in FIGURE 2. Under these conditions substantially all of the D.C. current applied from the D.C. signal source flows through the resistor 28, the voltage of the D.C. signal source being reduced by the small voltage of voltage source 29. During the opposite half cycle of operation, which is represented by FIGURE 4 the transistor '13 becomes conductive, and therefore, is represented by a fixed potential 33 while the transistor (17 becomes substantially non-conductive and therefore is represented by a very small constant current generator '34. Duringthis condition of opera tion, only a very small current flows through the load resistor 28', as determined by the voltage across the transistor 13, or referring to FIGURE 4, the voltage of the source 33. Therefore, the operation of the system is such that a large current flows through the resistor 28 in the cycle of operation illustrated in FIGURE 3, while a very small current flows through the same resistor during the operation illustrated in FIGURE 4.

By providing A.C. coupling to the load resistor 28 an alternating current is derived having a modulation envelope which is a function of the D.C. signal applied across the terminals 8 and 9 of the circuit illustrated in FIGURE 2. Difiiculties exist, in operation of the circuit, illustrated in FIGURE 2, as a result of variations of source impedance of the D.C. signal, that is, variations of the impedance of the resistor 32 illustrated in FIGURES pedances.

3 and 4. In order to exemplify the variations of operation with changes inthe impedance of the input circuit, assome in FIGURES 3 and 4 that the impedance 32 is reduced to zero:

Under these conditions the' current generator 31 in IGURE 3 and the current generator 34 in FIGURE 4 would be effectively shorted out and the voltage generators 29 and 33 would be applied across small im- In, the circuit of FIGURE 4, if the impedance 32 increases to infinity, the current flowing through the load resistor 28will not change. In the circuitof FIG- URE 3, when the impedance 32 is zero and the current sourcev is elfectively shorted out, substantially no current from the source 31 will flow through the load resistor 28. Ifthe impedance 32 increases to infinity, all the current form the source 31 will flow through the load resistor 28. Thusit can be seen that in FIGURE 3 the oamount of current from the current source 31 flowing through the load resistor 28 will vary substantially with changes in the impedance 32. Therefore, in the part. of the cycle represented by FIGURE 3 when the transistor 13 is cut ofi, the impedance of the D.C. signal source applied across terminals 8 and 9 will have a substantial effect upon the current flowingthrough the load resistor 28. In a quality instrument such variation cannot be tolerated.

In accordance with the invention, reference being now made to FIGURE 5 of the accompanying drawings wherein elements common to FIGURES 2 and 5 bear the same reference numeral, a constant current source is provided to buck out the current generated by the transistor 13 during its cut-off period. The constantcurrent source comprises a standard cell 36 having connected thereacross a resistive element 37 of a potentiometer 38, also having a variable tap 39, The lower ends, as viewed in FIGURE 5, of the resistor 37 and battery 36am connected in common to the lead 14, while ihCIZtP 39. is connected via an impedancedl to lead 11. The tap 39 is adjusted so that the current supplied by the circuit is exactly equal to the current through the transistor 13 during its cut-ofi interval and is also opposedthereto, so' that the two currents precisely cancel out. In'consequence, the currenttlow through the load 48 is unafiected' by source impedance change.

A variable resistor 44 is inserted in the lead 11 to provide an appropriate input impedance for the circuit, the impedance v44 being a value sufficiently large that changes in modulator impedancewill have little eifect on total input impedance. Likewise this value is small enough that changes in source or attenuator impedance will not materially reduce the effectiveness of the bucking current generator.

Directly shunting both transistors is resistor 42 and capacitor 43. Resistor 42 serves as a relatively low impedance path, with reference to resistance 44 and'any large source impedance. across input terminals, 8 and 9, for the current generated by transistor 13 during its cutoff period. Capacitor 43 serves as a low impedance A.C. path to the current generated by the transistor 13 during its cut-otf periods so as to minimize zero shift due to the residual A.C. voltages produced.

In addition to the elements previously discussed, with respect to the modulator 2,.there is provided a fixed resistor 46 and a variable resistor47 connected in series between the emitter electrode 12 and base electrode 18 of the transistor 13. The purpose of the resistors 46 and 47 becomes apparent upon a discussion of the graphs provided in FIGURES '68 of the accompanying drawings. FIGURE 6 is a graph of the A.C. output voltage of the modulator 2 plotted against the D.C. input voltage, in the absence of the resistors 46 and 47. It will be noted that not pass through the origin, or in other words, an A.C.v

output voltage is produced even though there is no D.C input voltage. The resistors 46 and 47 are employed. to

correct the first of theseconditions, whereas the bucking to assure that a linear portion of'the curve subsists to the right of the line, and therefore, the system should receive only positive D.C input voltages. Negative D.C.-

input voltages would obviously involve operation of the modulator on a non-linear portion of the curve and a portion of the curve having a negative slope. If the instrument is to be capable of accepting both positive and negative input voltages, the AC. output characteristic should be shifted so that the linear portion of the characteristic is centered on the vertical axis, that is, centered on the zero D.C. input axis. a

The shifting of the output characteristic of the modulator 2 by the resistors 46 and 47 will now be described. It is apparent that resistors 46 and 47 provide a continuous circuit through the load impedance, which, in the circuit illustrated in FIGURE 5 is a primary winding 48 of the transformer 49. and a capacitor 51 connected in series. The current flowing through the transformer primary 48 occurs only when transistor 13 is in a nonconducting state. Therefore, the signal developed across the transformer primary winding 4%, in consequence of the operation of the modulator, has superposed thereon a half wave A.C. signal, which is a function only of the impedance of resistors 46 and 47. As is indicated above, the signal produced as a result of the modulating action is a half-wave signal, this being produced only when the transistor 17 is conductive. The addition of an in-phase half wave voltage, due to resistors 46 and 47, to the halfvvave voltage developed by the modulating action, shifts the center line of the A.C. signal and in consequence shifts the characteristic curve, illustrated in FIGURES 6-8 with respect to the zero input signal axis. The amount of the shift of the characteristic is determined by the impedance of the variable resistor 47, a maximum shifting effect being achieved by reducing the value of resistor 47 to zero.

Referring again to the fact that the characteristic of the modulator 2 does not pass through the origin and that the bucking circuit 7 is employed to overcome this deficiency, reference is now made to FIGURE 9 of the accompanying drawings, which illustrates the demodulator circuit 5 and the bucking current generator 7. The AC. signal developed by the alternating current amplifier 4 of FIGURE 1 is applied to a base electrode 52 of a PNP transistor connected in the grounded-emitter configuration. The transistor is provided w-ith an emitter 54, which is coupled to a source of positive voltage through a bias resistor 56 shunted by a single by-pass capacitor 57. The transistor '53 is further provided with a collector 5 8 connected via a lead 59 to the recorder 6. The lbase-to-emitter circuit of the transistor supplies the required rectifying action for demodulation, while the carrier is by-passed by capacitor 57. The grounded-emitter connection of the transistor 53 is employed since such a circuit has a high output impedance. This feature of the circuit provided the required high impedance for damping the galvanometer mechanism of galvanometer type recorder 6.

Referring further to the circuit diagram of FIGURE 9 of the accompanying drawings, a collector electrode 61 of an NPN transistor 62 is connected to the lead 59 of the recorder 6. The transistor 62 is provided with a base electrode 63 connected to a source of fixed potential and is also provided with an emitter electrode 64 connected through a variable resistor 66 to a source of negative potential. 7 elements constitute the bucking current generator 7. er FIGURE 1. The variable resistor 66 is employed to ad just the current flowing through the transistor 62 so that it may precisely cancel the current applied to the recorder 6 through the transistor 53, when there is no input signal applied to the, system, as illustrated in'FIGURE 1 of the accompanying drawings. This adjustmentmay be made while the input to the system is short-circuited. The arrangement of the PNP and 'NEN transistors 53 and 62, respectively, is employed so that thetransistors may be connected directly in series, and tlilereforie, may effect direct series opposition of the currents produced by each.

Continuing with the description of the various components of the system of the invention, reference is made,

to FIGURE 10 wherein oscillator 25 of FIGURE 1, is illustrated in detail. The oscillator circuitiis a phasfi shift oscillator employing a transistor 6 .7 .havingcollector-l to-base feed-back. The transistor 67 is connected in a grounded-emitter circuit that includes an emitterelectrode 68, a collector electrode 69 and a base electrode 71. The emitter electrode 68 is connected to ground through a bias resistor 72., which is shunted by a lay-pass capacitor 73 and the collector electrode 6$? is connected to the lower end of a primary winding 74 of an output trans} former '76. The upper end of the primary'winding 74, as viewed in FIGURE 10, is connected via a'lead'77 to a regulated negative voltage'supply. The collector elec' trode 69 of the transistor 67 is connected through a phase-shift circuit generally designated by the reference numeral 82, connected in series, to the base electrode 71 of the transistor 67. The path through the phase-shift circuit 82 constitutes a positive phase-shift feed-back circuit which supports oscillations in the circuit. The lead 77 is connected through a resistor 33 and capacitor 84 to ground and the junction of these two elements is com nected through a resistor 85 to the base electrode 71 of the transistor 67. The circuit, as thus far described, operates as an uncompensated phase-shift, collector-to-base feed-back, grounded emitter oscillator,

Conventional transistor oscillators are subject to amplitude variations with temperature because the gain of the transistor increases with an increase in temperature. Obviously, such operation cannot be permitted in a highquality instrument. In order to stabilize the amplitude of the oscillator, illustrated in FIGURE 10, against changes in temperature, there is employed a temperature sensitive diode 86 connected directly across the primary winding 74 of the output transformer 76, with its cathode connected to the collector electrode of the transistor 67. The diode serves to limit the voltage swing of the collector electrode when the swing is in the forward conducting direction of the diode only. Specifically, when the collector electrode 69 swings negative with respect to the zero or reference voltage, the diode is rendered con ductive after this swing has overcome the back bias on the diode developed by the negative steady-state current drawn through the transformer primary winding 74. When the diode 86 is rendered conductive, it diverts a portion of the current drawn by the transistor, as determined by the relative impedances of the diode and transformer winding. The temperature characteristic of the diode 86 is such that its impedance decreases with temperature, so that as the gain of the transistor 67 increases with temperature, an increasingly large percentage of the current supplied by the transistor 67 is diverted through the diode 36. Therefore, in spite of an increase in cur rent through the transistor there is not a corresponding increase through the transformer winding 74, the apparent impedance of the diode 86 and transformer winding 74 being reduced as the impedance of the diode 86 is reduced.

Due to the relatively long time constant of the feed- The transistor 62 and its associated circuit back network, that is, of the phase-shift circuit 82 and the capacitor 81, the clipping action aifected by the diode 86 is so slight as to produce a negligible efiect on the mum number of components and a minimum of circuit,

complexity.

Although the invention has been shown and described in terms of a preferred embodiment, it will be appreciated that various change and modifications can be made which do not depart from the inventive concepts as expressed in the, claims that :follow. 7

What is claimed is:

1. A modulator comprising two transistors each having collector, emitter and base electrodes, means for applying a direct voltage-across said emitter electrodes, a i

center-tapped source of alternating signals connected between said base electrodes, said collector electrodes being connected together and to the center tap of said source, a load impedance connected between said collector electrodes and said emitter electrode of one of said transis tors, and a variable resistor connected between the base electrode and the emitter electrode of said one transistor.

2. A modulator comprising two transistors each having a collector an emitter and a base electrode, means for applying a direct voltage across said emitter electrodes, a bucking current source, and source being connected across said emitter electrodes andadjustable to be equal and of reverse polarity to the cut-off current of one of said transistors, a center-tapped source of alternating signals connected between said base electrodes, said collector electrodes being connected together and to the center tap of said source of alternating signals and a load impedance connected between'said collector electrodes and said emitter electrode of said one of said transistors.

References (lite-d by the Examiner UNITED STATES PATENTS 4/58 Raisbeck 332-43 OTHER REFERENCES ROY LAKE, Primary Examiner.

L. MILLER ANDRUS, ALFRED L. BRODY, Examiners. 

1. A MODULATOR COMPRISING TWO TRANSISTORS EACH HAVING COLLECTOR, EMITTER AND BASE ELECTRODES, MEANS FOR APPLYING A DIRECT VOLTAGE ACROSS SAID EMITTER ELECTRODES, A CENTER-TAPPED SOURCE OF ALTERNATING SIGNALS CONNECTED BETWEEN SAID BASE ELECTRODES, SAID COLLECTOR ELECTRODES BEING CONNECTED TOGETHER AND TO THE CENTER TAP OF SAID SOURCE, A LOAD IMPEDANCE CONNECTED BETWEEN SAID COLLECTOR ELECTRODES AND SAID EMITTER ELECTRODE OF ONE OF SAID TRANSISTORS, AND A VARIABLE RESISTOR CONNECTED BETWEEN THE BAE ELECTRODE AND THE EMITTER ELECTRODE OF SAID ONE TRANSISTOR. 